[sdiy] Dual/quad opamp bleed-through

[Dave Manley]

Oakley Sound wrote:
> > Do any of these synths have unintentional oscillator sync problems?
>
> Minimoog; no, in any of the three variants. But there is a belief that 
> the power supply in the Mini is noisy enough so as to disrupt any soft 
> syncing between the VCOs.
Interesting.  FWIW, I took a look at the Voyager and it also has all the 
VCOs on the same board, I assume it has a clean power supply.  So 
placement and routing must be the key factor.  I presume the LP is the 
same, but I've never seen the internals.  Anybody have a link to some LP 
pcb images?
>
> ARP 2600; no. But this is because all the VCOs, although sharing the 
> same board, have completely separate power supply lines back to the 
> power supply module. ARP must have known there was a potential problem 
> to take the hassle of three extra Molex headers and associated wiring.
Yeah, except I specifically didn't mention the 2600 because the 
oscillator cores are in submodules that are mounted to the main VCO 
board, and as you mention each VCO has separated power.    Also, I've 
heard at least one person (perhaps Drew Neumann on AH) mention soft-sync 
on the 2600s - I haven't heard anyone else though.  On the Odyssey 
(2800) the oscillator modules were removed and integrated into the VCO 
board, and the power is distributed on the pcb.
>
> MS-20; yes. Soft sync is evident on many units I have seen. But only 
> when you try to set the pitches very close to one another.
>
Of the three (Mini, Odyssey, MS-20), the MS-20 has the most dense parts 
placement, while the Odyssey has a lot of board area to work with due to 
all the slide pots and switches determining the pcb size.  In the end, 
having multiple VCOs on the same pcb is possible, but you may have to 
enlarge the pcb size, and might encounter more than a little pain to 
engineer out any coupling issues.

-Dave